The present applicant developed, and now manufactures and sells, a hybrid car called the HIMR which makes combined use of an internal combustion engine and an electric motor. In this vehicle, a three-phase alternating current squirrel-cage induction machine is coupled to the crankshaft of an internal combustion engine, a large battery is mounted on the vehicle, a bidirectional inverter is coupled between this battery and alternating current induction machine, and this inverter is controlled by a program control circuit (see WO88/06107).
With this system, when the vehicle accelerates, the rotating magnetic field applied to the alternating current induction machine is controlled so that the alternating current induction machine constitutes an electric motor, and when the vehicle decelerates, the rotating magnetic field applied to the alternating current induction machine is controlled so that the alternating current induction machine constitutes a generator. The control performed by this system is such that the battery discharges when the alternating current induction machine is utilized as an electric motor, and the battery charges when it is utilized as a generator. In other words, this system is controlled to provide regenerative braking.
This system has been mounted on large buses and has been applied to buses on urban routes and to buses operating in regions where it is essential to keep pollution extremely low. In recent years, pollution from the exhaust of vehicles with internal combustion engines has become a major problem, and this has led to discussion of the possibility that most vehicles being run in urban areas will become electric vehicles, despite the higher price of such vehicles and the fact that their fuel is quite expensive.
The HIMR described above is arranged to provide a battery compartment in the vehicle, to use batteries with a terminal voltage of 12 V as unit cells, such batteries being mass produced and therefore inexpensive to procure, and to mount twenty-five of these in the battery compartment and connect them electrically in series to give an overall terminal voltage of 12 V.times.25=300 V. This arrangement is utilized as a battery for supplying energy for running the vehicle.
The term "unit cell" here signifies the unit which, when a multiplicity are connected in series, comprises the battery for supplying energy to run the vehicle. For example, although in the case of a lead storage battery the chemical properties dictate that the terminal voltage of the smallest unit cell is 2 V, a commercial battery generally comprises a plurality of these 2 V cells connected in series and housed in one casing. For example, in the case of a lead storage battery, the terminal voltage of the unit cell can be 2 V, 4 V, 6 V, 12 V, 24 V and so forth. For other kinds of battery, the terminal voltage of the unit cell is determined by the chemical properties of the cell and by the number of cells connected in series.
The present applicant has filed an International Patent Application (see PCT/JP96/00966 and WO96/32651) relating to the monitoring of unit cells.
This system has voltage detection circuits which detect when the voltage between the positive terminal and the negative terminal of a multiplicity n (for example, 25) of unit cells connected in series by connection cables is less than a preset value. These voltage detection circuits comprise a first light-emitting display circuit which indicates by emitting red light when the voltage between the positive terminal and the negative terminal is less than a first preset value (V.sub.1), as an indication that charging is required, and a second light-emitting display circuit which indicates by emitting green light when the voltage between the positive terminal and the negative terminal exceeds a second preset value (V.sub.2), as an indication that the unit cell is charged. These light-emitting displays of the voltage detection circuits serve to give notification of the state of charging of the unit cells, i.e., of whether the unit cells are poorly charged or well charged.
The present inventor has been able to obtain a large number of records relating to the running and maintenance of the aforementioned HIMR vehicles. Because batteries gradually deteriorate with repeated charging and discharging, they have to be replaced after a certain time. However, in the prior art battery life was not taken into consideration in the control of charging and discharging. In other words, hitherto charging and discharging control of the battery of an electric vehicle has been performed in accordance with the running conditions of the vehicle, but without communicating the present state of the battery to the control system and taking this battery state into consideration.
This will now be explained in greater detail. If it is supposed for example that the standard terminal voltage of the battery is 300 V as described above, then there is a danger of damage to the battery if its terminal voltage exceeds a prescribed limit (e.g., 380 V) when charging by regenerative braking. Hitherto, therefore, control has been performed to ensure that this limit is not exceeded. However, this limit is actually a safe value for when the battery is well charged, whereas if a battery is in good condition (when it has not deteriorated and is not well charged), it could be efficiently charged at an even higher terminal voltage. This means that with a new battery which is not well charged, when the brake pedal is depressed and the vehicle is being braked, the amount of energy which ends up being dissipated as frictional heat of the brake shoes can be reduced, and more energy can be utilized for recharging the battery by regenerative braking.
In a hybrid vehicle, similar considerations apply when the accelerator pedal is depressed and the vehicle is accelerating. Namely, during acceleration the required torque is provided jointly by the internal combustion engine and the electric motor, but pollution can be reduced by applying control such that, at a given amount of accelerator pedal displacement (i.e., for a given torque), if the battery is in good condition (when it has not deteriorated and is well charged), the discharge current is increased and the burden on the internal combustion engine is reduced. If the battery is not well charged (when its deterioration has advanced or it is poorly charged), it is anticipated that the burden on the battery can be reduced and the burden on the internal combustion engine increased.
More detailed observations have shown that if for example twenty-five unit cells are discharged when connected in series, energy is not released uniformly from all twenty-five unit cells. Furthermore, when charging series-connected unit cells, not all the cells are charged uniformly. This is easily understood in terms of electrical characteristics by assuming that the internal resistance (R) of the individual unit cells is not equal. Because the unit cells are connected in series, the current (I) will be equal, but during charging and discharging the charging or discharging energy per unit time (I.sup.2 R) will not be equal. A unit cell with a higher internal resistance will have a higher terminal voltage during charging than other unit cells, and conversely will have a lower terminal voltage during discharge than other unit cells. Even assuming that the terminal voltages are actually equal, if all the unit cells are repeatedly charged and discharged at a standard or rated voltage, a cell with a high internal resistance will end up being overcharged during charging, with the result that this unit cell alone will undergo accelerated deterioration. Furthermore, despite being charged and discharged on the basis of series connection, a unit cell with a high internal resistance will acquire an elevated cell temperature, with the result that its characteristics will differ from those of other unit cells, and again this unit cell alone will end up deteriorating before other unit cells.
In other words, it has been discovered that an important factor for extending battery life is to determine the maximum value for the charging current or the maximum value for the discharge current not just in accordance with the state of the overall battery but also in accordance with the state of individual unit cells.
The inventor has carried out various trials such as housing unit cells from the same production lot in one battery compartment. It was discovered that even if the characteristics of the unit cells in a new vehicle are uniform, when the vehicle has been used for a long period of time these characteristics exhibit variability, and non-uniform deterioration accelerates. In general it is not individual unit cells of a battery which are replaced, but rather all the unit cells are replaced simultaneously. It is therefore evident that employing uniform conditions in the control of an entire battery is a cause of shortened battery life. Moreover, the use and subsequent disposal of large numbers of batteries will constitute a new source of pollution.
Against such a background, it is an object of the present invention to provide a system capable of observing the state of charging and discharging of a battery, and of assisting the driving gear and charging the battery in accordance with this state. It is another object of the present invention to perform control of charging and discharging which is suited to the battery, not just according to the state of charging of the battery, or in other words not just according to whether the battery is poorly charged or well charged, but also in accordance with whether the battery is new or is in a state of advanced deterioration. It is a further object of the present invention to improve battery charging and discharging efficiency and to lengthen the working life of a battery. It is yet another object of the present invention to reduce the amount of energy lost by the brakes and to regenerate as much energy as possible. It is a further object of the present invention to provide a control system such that even when there is variability in the characteristics of the unit cells, this variability is not made larger by long-term use. It is another object of the present invention to decrease battery costs for electric vehicles. It is still another object of the present invention to simplify battery maintenance.